A disk storage system is utilized to write new data on old data which become unnecessary on a magnetic disk as a magnetic recording medium. Thereby, old data are erased and also new data are recorded. Accordingly, a magnetic field generated from a magnetic head used for writing new data necessitates at least a magnitude sufficient for writing data onto a total thickness of the recording medium. When the magnetic field generated from the magnetic head is weak so that new data can not be written onto the total thickness of the recording medium, old data remain thereon without being erased and remain recorded in overlapping relationship with new recorded data.
For this reason, the magnetic field strength generated at the magnetic head used for the disk storage system is evaluated by an overwrite characteristic indicative of the writing ability thereof.
The overwrite characteristic is shown by a ratio of reproduced output of new data to that of the remaining old data which has not been erased on a magnetic disk.
Practically, the overwrite characteristic is during the manufacturing process of the magnetic head wherein data written by the lowest frequency among modulated signals used by the disk storage system is used as old data and other data written by the highest frequency thereamong is used as new data for erasing the old data. A current value of the signals for exciting the magnetic head is utilized for both writing old data and new data.
Suppose that a reproducing output value by the nonerased old data which was written by the lowest frequency is .DELTA.E1f, and a reproducing output value of the new data written by the highest frequency is E2f; the value OW of the overwrite characteristic is represented by a decibel expression of these ratio as follows: EQU OW=-20 log E2f/.DELTA.E1f (dB)
A value of the overwrite characteristic of a magnetic head used as a product is approximately -25 dB.
Recently, in the disk storage system, the thin film magnetic head is required to be one for increasing recording density to a recording medium and attaining a large memory capacity.
In the thin film magnetic head, the magnetic field strength generated depends on the shape of the tip portions of the magnetic core, especially, depends largely on the throat height.
Conventionally, many attempts have been made with respect to methods for adjusting the throat height for producing the thin film magnetic head having a writing characteristic in such a manner that necessary OW value is obtained.
For instance, such an idea is disclosed in FIGS. 1 and 2 of Japanese Patent Laid-Open No. 60-254404 published on December 16, 1985 having the title of "Manufacturing method of thin film magnetic head".
However, in accordance with this disclosure a thin film magnetic head is manufactured in such a manner that the track width TW, the pole lengths P.sub.1, P.sub.2 (the thicknesses of the magnetic layers forming the magnetic gap portion), and the magnetic gap length Gl (the thickness of the non-magnetic layer forming the magnetic gap portion) thereof are small. In this case, only by forming the throad height to a predetermined size, the writing characteristic or ability thereof can not be properly determined. Namely, the writing characteristic is affected by the total configuration at the tip portions of the magnetic head.
Further, in the above-explained thin film magnetic head, when the throat height is decreased, for instance, to approximately 0.3 .mu.m as well as in the case of the track width, etc., the tip portions of the magnetic head cannot be measured by an optical method.
For that reason, the judgment whether the writing characteristic or ability of the thin film magnetic head manufactured is good or not, has to be carried out by measuring the OW value after the manufactured thin film magnetic head is assembled into the disk storage system, and the recording and reproducing thereof are operated.